This invention relates to an aspherical glass lens element for use in an optical system of a disk device which may be, for example, a laser disk device, a disk device for a computer memory, a magnetic optical disk device, or the like and, in particular, to an aspherical glass lens element which is used for a collimator lens system in the disk device.
In general, a collimator lens system of the type described confronts a light source, such as a semiconductor laser, in an optical pickup device of a laser disk device and is for use in combination with an object lens system which is adjacent to an optical disk. Supplied with divergent rays from the light source, the collimator lens system serves to convert the divergent rays into parallel or collimated rays of a large aperture which is directed to the object lens system and which is focused on the optical disk. In this event, a degree of parallelization of the parallel rays should be strictly controlled because the degree of parallelization directly influences an operation of the laser disk device.
Herein, it is to be noted that an oscillation wavelength of a semiconductor laser is changed to another as an input current supplied to the semiconductor laser is varied from one to another. Such a variation of an input current brings about a variation of a chromatic aberration and results in degradation of the degree of parallelization of the parallel rays emitted from the collimator lens system.
Alternatively, consideration is made as regards an optical pickup device which is used in an optical rewritable disk device. In this case, write-in and readout operations are carried out in such an optical rewritable disk device by the use of a semiconductor laser. Inasmuch as such a semiconductor laser should raise up output power on the write-in operation in comparison with the readout operation, different oscillation wavelengths are generated from the semiconductor laser. Under the circumstances, the degree of parallelization of the parallel rays is undesirably degraded in the optical rewritable disk device also.
In order to avoid such degradation of the degree of parallelization and to correct the chromatic aberration, a combinational lens system is used as the collimator lens system. Specifically, such a collimator lens system is formed either by a first achromatic lens composed of a negative meniscus lens and a double-convex lens adhered to the negative meniscus lens or by a second achromatic lens composed of a pair of single lenses spaced apart from each other.
However, adherence of the negative meniscus lens to the double-convex lens and alignment of both the negative meniscus lens and the double-convex lens are required on manufacturing the first achromatic lens. On the other hand, it is very difficult to avoid decentration of the single lenses through an interval of space in the second achromatic lens.
As a result, the lenses and the lens mount must be precisely machined or processed on manufacturing the lenses and the lens mount. This makes it difficult to assemble the lenses and the lens mount and results in an increase of costs.
On the other hand, a plastic aspherical lens element is manufactured by injection molding and sold as the collimator lens system. By way of example, such an aspherical lens element is disclosed by Arai et al in U.S. Pat. No. 4,449,792, and has a large aperture, a long back focal length and a long working distance. The aspherical lens element disclosed by Arai et al may be used as a pickup lens for video disks and has an entrance or first surface and an exit or second surface both of which are defined by first and second predetermined formulae, respectively. Besides, a first factor of (r.sub.1 /(n - 1)f), a second factor of k.sub.1, and a third factor of k.sub.2 are selected within predetermined ranges, respectively, where r.sub.1 represents a radius of curvature of the first surface; n, a refractive index of lens; f, a focal length of lens; and k.sub.1 and k.sub.2, coefficients of cones of the first and the second surfaces.
However, it has been found out that the plastic aspherical lens element causes the chromatic aberration to occur when it is used as the collimator lens for the optical disk device such that wavelengths are varied from each other in the write-in and the readout operations. In consequence, the degree of parallelization is degraded in the parallel rays.
Moreover, the plastic aspherical lens element is inferior to a glass aspherical lens element in a heat resistance, homogeneity, humidity durability, and a temperature characteristic. In addition, no consideration is made in Arai et al about using such a plastic aspherical lens element as a collimator lens of the optical disk device.